Reversal of Persistent Fibrosis in Aging by Targeting Nox4-Nrf2 Redox Imbalance

Abstract

The incidence and prevalence of pathological fibrosis increase with advancing age, although mechanisms for this association are unclear. We assessed the capacity for repair of lung injury in young (2 months) and aged (18 months) mice. While the severity of fibrosis was not significantly different between these groups, aged mice demonstrated an impaired capacity for fibrosis resolution. Persistent fibrosis in lungs of aged mice is characterized by the accumulation of senescent and apoptosis-resistant myofibroblasts. These cellular phenotypes are sustained by alterations in cellular redox homeostasis resulting from elevated expression of the reactive oxygen species (ROS)-generating enzyme, NADPH oxidase-4 (Nox4), and an impaired capacity to induce the NFE2-related factor 2 (Nrf2) antioxidant response. Lung tissues from human subjects with idiopathic pulmonary fibrosis (IPF), a progressive and fatal lung disease, also demonstrate this Nox4-Nrf2 imbalance. Nox4 mediates senescence and apoptosis resistance in IPF fibroblasts. Genetic and pharmacologic targeting of Nox4 in aged mice with established fibrosis attenuated the senescent, anti-apoptotic myofibroblast phenotype and led to a reversal of persistent fibrosis. These studies support the concept that loss of cellular redox homeostasis promotes pro-fibrotic myofibroblast phenotypes that result in persistent fibrosis associated with aging. Importantly, our studies suggest that restoration of Nox4-Nrf2 redox balance in myofibroblasts may be an effective therapeutic strategy in age-associated fibrotic disorders, potentially to resolve persistent fibrosis or even reverse its progression.

Mechanisms of Lung Fibrosis Resolution

“…A transient senescence response was observed in fibroblasts from young mice with resolving bleomycin-induced fibrosis supporting a role for senescence in spontaneous lung fibrosis resolution. 51 However, there are accumulating data supporting the fact that prolonged activation of senescent pathways contributes to persistent pulmonary fibrosis. Pulmonary fibrosis is classically, but not exclusively, a disease of aging.…”

“…91 Although the aged lung is more susceptible to the onset of fibrosis, few studies in the lung or other organs specifically address the capacity for resolution of established fibrosis in aged models. Hecker et al 51 demonstrated that bleomycin-induced fibrosis resolution was impaired in aged mice. In contrast to younger mice where the senescence response was transient, persistent fibrosis in the aged mice was associated with a prolonged senescent and apoptosis-resistant phenotype of myofibroblasts.…”

“…Genetic and pharmacological targeting of Nox4 in aged mice after fibrosis enhanced the capacity for fibrosis resolution, supporting the concept that restoration of Nox4-Nrf2 reduction-oxidation balance in myofibroblasts may be a therapeutic strategy to resolve persistent fibrosis or even reverse its progression. 51 In summary, although transient senescence may play an important role in myofibroblast elimination in normal wound healing, prolonged senescence of myofibroblasts in the lung induces resistance to apoptosis and may be a primary mechanism of persistent fibrosis. Supporting this hypothesis, Baker et al, 80 using transgenic approaches in mice allowing for the inducible elimination of cells carrying the p16 senescence biomarker, demonstrated that life-long removal of senescent cells delays the onset of age-related phenotypes.…”

Nuclear Factor–Erythroid-2–Related Factor 2 in Aging and Lung Fibrosis

“…Exposure of lungs to various endogenous and exogenous oxidants may elicit inflammatory and fibrotic responses, leading to impaired antioxidant capacity. 2 This may perpetuate the injury response and impede tissue regeneration with sustained fibrosis. This oxidant-antioxidant imbalance may be accentuated in aging.…”

“…112 More recent studies demonstrate that the up-regulation of NOX4 is coupled to Nrf2 induction, and this adaptive response is critical to apoptosis susceptibility of myofibroblasts and resolution of fibrosis in young mice. 2 In contrast, in aged mice, there is impaired Nrf2 activation leading to an altered NOX4-Nrf2 redox balance and acquisition of an apoptosis-resistant myofibroblast phenotype resulting in persistent fibrosis. 2 This supports a mechanism for the antagonistic pleotropic action of NOX4 that is determined by the activation.…”